The present invention relates to the desulfurization of carbonaceous material, and more particularly to a method for the removal of organic and/or inorganic sulfur from carbonaceous material by treatment of the material with a metal carbonyl or a low valent complex of the transition metals under alkaline conditions in the presence of water.
The present world-wide emphasis on the energy crisis has resulted in increased attention by both governmental and private organizations to the need for readily available and environmentally acceptable energy sources. One of the major problems in the utilization of some carbonaceous materials, such as some coals, crude petroleum oils, primary refinery products, and the like, occurs when the carbonaceous material has a relatively high sulfur content. It has been found that combustion of high sulfur content carbonaceous materials can result in the formation of compounds, like sulfur oxides, which have been found to be environmentally undesirable when present in large quantities.
For example, the United States has vast resources of coal for development as a potential energy source. Depending on their origin, coals contain varying amounts of sulfur, which may be present, for example, as elemental sulfur, as iron disulfide or pyritic sulfur, as sulfate sulfur, or as organic bound sulfur. The presence of sulfur in coal is a tremendous disadvantage to the use of the coal as an energy source, particularly in view of the emphasis on pollution control as illustrated by federal emission control standards for sulfur dioxide. Illustrating the enormity of the sulfur dioxide emission problem is the fact that large transportation expenses are incurred by coal users in transporting Western and European coal of relatively low sulfur content long distances to supplant available high sulfur-containing coals in order to make compliance with sulfur dioxide emission standards possible when using coal as an energy source. There has been a longfelt need for an economically and commercially feasible means for absorbing the large amounts of sulfur dioxide emitted by the combustion of coal. Currently, U.S. utilities in burning about 395 million tons of coal a year generate about 21 million tons of sulfur dioxide in the process.
In other carbonaceous materials, such as crude petroleum and primary refinery products, sulfur and some sulfur-containing compounds can be highly corrosive to engine components when present in fuels, can form combustion products which can be highly corrosive and/or can cause sulfur poisoning in catalytic processes, particularly where cobalt, nickel, and related catalysts are employed.
Various methods for removing impurities such as sulfur from raw coal and other carbonaceous materials have been extensively tested over the years. Among these are methods which employ the difference in specific gravity between coal particles and non-coal particles, or differences in their surface, electrostatic, chemical, or magnetic properties. For various reasons, difficulties are encountered in making an efficient gravity separation of impurities from coal which has been ground sufficiently fine to substantially liberate non-coal particles from coal particles. In water systems this difficulty is typically related to the slow settling rate of fine particles, and in air systems to the large difference in specific gravity between air and particles. However, some success in the removal of pyritic sulfur, ash-forming minerals, and various sulfates has been obtained by processes which enhance the magnetic susceptibility of the pyrite, ash-forming minerals, or sulfates, thereby permitting removal of these impurities by magnetic separation means. For example, U.S. Pat. Nos. 3,938,966, 4,098,584, 4,119,410, 4,120,665 and 4,175,924 relate to such processes. In addition, it has been suggested in U.S. Pat. No. 4,119,410 that heat pretreatment may be used to remove elemental sulfur from coal. While the foregoing processes have been effective in the removal of pyritic sulfur and other impurities from coal by enhancing magnetic susceptibility, there is no suggestion in these patents that such processes have any effect on the organic bound sulfur of carbonaceous material or on the removal of inorganic sulfur by other than magnetic means.
Organic sulfur in carbonaceous material is typically bound in the material in the form of sulfides, disulfides, mercaptans, thiophenes, derivatives thereof and the like. It has been suggested in U.S. Pat. No. 4,146,367 of Hsu that a portion of the organic bound sulfur may be removed from coal by slurrying the coal in an aromatic hydrocarbon solvent containing iron pentacarbonyl and then heating the slurry to a temperature of above 40.degree. C. but less than 150.degree. C. for from 1 to 10 hours. Due to the nature of sulfur containing coal, reaction under the conditions disclosed by Hsu is characteristically conducted in acidic media. The reaction of iron carbonyl with organic bound sulfur under the foregoing conditions is disclosed by Hsu to involve the desulfurization of thiophene compounds resulting in the formation of carbonyl sulfide and compounds containing iron and sulfur. In addition to the foregoing, U.S. Pat. No. 3,996,130 of Nametkin et al. discloses reducing the sulfur, nitrogen and oxygen content of crude petroleum and primary refinery products by treatment with .pi.-complexes of transition metals such as iron carbonyl, salts of platinum metals such as Na.sub.4 PtCl.sub.4 and Na.sub.2 PtCl.sub.4, or .pi.-allylic complexes of platinum group metals at a temperature of from 80.degree. to 120.degree. C., followed by treatment with a chelating agent to react with unconverted organometallic compounds, and finally by separation of the target compounds from the reaction mixture, such as by distillation.
It has now been determined that removal of sulfur from carbonaceous materials, such as coal, crude petroleum, primary refinery products and the like, can be obtained by contacting the carbonaceous material with an agent selected from the group consisting of metal carbonyls, other low valent complexes of the transition metals, and mixtures thereof, and water under alkaline conditions, and heating the mixture for a sufficient period of time to obtain sulfur removal from the carbonaceous materials. When the agent is iron carbonyl, for example, reaction of the sulfur with iron carbonyl at alkaline pH levels appears to involve formation of iron tetracarbonyl hydride anion, HFe(CO).sub.4.sup.-, and/or iron tetracarbonyl dihydride, H.sub.2 Fe(CO).sub.4, which appear to be more reactive with the sulfur of the carbonaceous material than is iron pentacarbonyl, Fe(CO).sub.5, of the processes disclosed in the Hsu and Nametkin et al. patents.